The methods for producing various types of alcohol from grain generally follow similar procedures, depending on whether the process is operated wet or dry. One alcohol of great interest today is ethanol. Ethanol can be produced from virtually any type of grain, but is most often made from corn.
Since its inception, the national market for fuel ethanol has grown from about 6.6 million liters (about 175 million gallons (gal)) in 1980 to about 7.9 billion liters (about 2.1 billion gal) in 2002. In 2003, the U.S. ethanol industry produced a record 10.6 billion liters (about 2.8 billion gal), all of which was produced from 74 ethanol plants located mainly within the corn-belt. Recent federal government legislation has been proposed, which would mandate that ethanol production capacity grow to approximately 1.9 trillion liters (approximately five (5) billion gal) by 2012. Consequently, ethanol producers are seeking methods to improve yields before incurring the high capital costs of direct plant expansion. Because of the ongoing need for ethanol, as well as recent and expected future rapid growth of the ethanol industry, producers are finding it difficult to incur the time and expense required to refine existing technologies to meet the potentially mandated increases and also remain cost competitive with intense ethanol producer competition. Higher yields are also desired for other types of alcohol.
Alcohols such as ethanol can be produced from virtually any type of grain, but ethanol in particular is most often made from corn, which contains high levels of starches that can be broken down into the glucose sugars needed for traditional fermentation. However, there is a growing interest in producing alcohol from other sources, such as cellulose, a linear polymer of glucose molecules. Cellulose is a desirable alternative over other ethanol feedstocks such as corn grain since it is renewable, abundant, does not take away from the food supply and is available at a relatively low cost. However, there are several known difficulties associated with efficiently converting the cellulose (contained in biomass) to glucose sugars, including the extensive chemical treatment required and the high capital and energy costs involved.
The biggest challenge for a commercial biomass-to-alcohol process is the ability to cost-effectively convert hemicellulose and cellulose to fermentable sugars. The combination of hemicellulose and lignin provide a protective sheath around the cellulose, which must be modified or removed before efficient hydrolysis of cellulose can occur. Furthermore, the cellulose must be decrystallized or “softened” before it can be processed into alcohol. Softening entails insertion of water into the crystalline structure of the cellulose, thereby opening up or loosening its structure such that it can be economically converted to glucose for fermentation.
The pretreatment softening process also usually includes hydrolysis of hemicellulose to pentose sugars, which precedes enzyme or acid hydrolysis of the cellulose to glucose. However, pretreatment-hydrolysis of plant biomass can often result in the creation and release of other chemicals that inhibit microbial fermentation. These inhibitors (i.e. furfural) are largely the product of sugar degradation, and methods to remove these inhibitors or to reduce their formation are needed.
Biomass conversion to alcohol also poses unique fermentation considerations. The Saccharomyces cerevisiae yeast strains used in conventional corn ethanol plants for example, can ferment glucose, but can not ferment pentose sugars such as xylose. Additionally, there is currently no naturally occurring microorganism that can effectively convert all the major sugars present in plant biomass to ethanol. Therefore, genetically engineered yeast or bacteria, which can ferment both glucose and xylose to alcohol are being used for biomass to alcohol processes. However, genetically-enhanced recombinant strains of fermentative microorganisms, including recombinant strains of yeast, bacteria and fungi, as well as transgenic nucleic acids (DNA, RNA) derived from such component may pose environmental disposal and permitting problems. Methods to remove these components in product and waste streams are needed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a significant need in the art for systems and methods that provide for improved biomass conversion to alcohol, such as ethanol, in a cost-effective manner.